Facile synthesis of anthranilic acid based dual functionalized novel hyper cross-linked polymer for promising CO2 capture and efficient Cr3+ adsorption

A novel hyper cross-linked polymer of 2-Aminobenzoic acid (HCP-AA) is synthesized for the adsorption of Cr3+ and CO2. The Brunauer–Emmett–Teller surface area of HCP-AA is 615 m2 g−1. HCP-AA of particle size 0.5 nm showed maximum adsorption of Cr3+ for lab prepared wastewater (93%) while it was 88% for real industrial wastewater. It is might be due to electrostatic interactions, cation-π interactions, lone pair interactions and cation exchange at pH 7; contact time of 8 min; adsorbent dose 0.8 g. The adsorption capacity was calculated 52.63 mg g−1 for chromium metal ions at optimum conditions. Freundlich isotherm studies R2 = 0.9273 value is the best fit and follows pseudo second order kinetic model (R2 = 0.979). The adsorption is found non-spontaneous and exothermic through thermodynamic calculations like Gibbs free energy (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were 6.58 kJ mol−1, − 60.91 kJ mol−1 and − 45.79 kJ mol−1 K−1, respectively. The CO2 adsorption capacity of HCP-AA is 1.39 mmol/g with quantity of 31.1 cm3/g (6.1 wt%) at 273Kwhile at 298 K adsorption capacity is 1.12 mmol/g with quantity 25.2 cm3/g (5 wt%). Overall, study suggests that carboxyl (–COOH) and amino (–NH2) groups may be actively enhancing the adsorption capacity of HCP-AA for Cr3+ and CO2.


Equipment
FTIR spectra were acquired using a Thermo Nicolet Nexus 670 spectrophotometer.Elemental composition analysis of the hyper cross-linked polymers (HCPs) was conducted through Energy Dispersive X-ray Spectroscopy (EDX).Scanning Electron Microscope (SEM) imaging and analysis were performed using a Zeiss Ultra-55 instrument.Transmission Electron Microscope (TEM) analysis was conducted with a JEM-2100 plus microscope.X-ray Diffraction (XRD) powder analysis of the synthesized polymer samples was carried out using a Smart Lab TM 3 kW X-ray diffractometer.Thermogravimetric analysis was carried out by using Netzsch Jupiter thermal analyzer.To determine the BET surface area, pore volume, pore size and nitrogen adsorption/ desorption isotherms of the HCP-AA, Micro-metrics ASAP 2020 M and porosity analyzer from Micrometrics, USA were used.

Characterization of HCP of anthranilic acid (HCP-AA)
Physical appearance Appearance of HCP-AA was amorphous and black in color that is shown in Fig. 1.

Physical properties of HCP-AA
Physical properties of HCP-AA are shown in Table 1, which shows that HCP-AA melts at temperature above 400 °C.www.nature.com/scientificreports/Chemical equation for synthesis of HCP-AA Chemical equation for the preparation of anthranilic acid based hyper cross-linked polymer is shown in Fig. 2.
Mechanism of synthesis of HCP-AA Figure 3 depicts the possible reaction mechanism for HCP-AA synthesis via Friedel Craft alkylation.

FT-IR results of HCP-AA
The FTIR spectrum of the HCP-AA is represented in Fig. 4. The peak, having medium intensity is shown at 1590 cm −1 that may indicates the presence of the N-H bond.The peak, having medium intensity which is observe at 1508 cm −1 , indicates the presence of the C=C bond stretching in benzene ring.The hydroxyl group of carboxylic acid also involved in hydrogen bonding this is indicated by the presence of weak peak at 1314 cm −1 the presence of C-N bond is indicated by the weak peak at 1213 cm −1 .Weak intensity peak at 1122 cm −1 indicates the presence of C-C bond of benzene ring.The most important and prominent peak at 715 cm −1 indicates the cross linking by C-Cl bond which may prove the synthesis of HCP-AA.
Energy dispersive X-ray spectroscopy HCP-AA have carbon, nitrogen, oxygen and chlorine elements.The percentage of carbon was 65.84% that was the highest percentage of all other elements.Similarly, it had nitrogen of about 24.76, oxygen 7.87%, chlorine 1.32% as shown in Table 2 and Fig. 5.It also contains aurum 0.22% that might be come at the time of coating during SEM analysis.

SEM and TEM results of HCP-AA
The TEM and SEM analysis (Figs. 6 and 7) suggests that HCP-AA had porous structure and due to presence of abundant pores, it had excellent surface area.BET surface area of HCP-AA is 615 m 2 g −1 .Both SEM and TEM results prove that these materials are very suitable for uptake of Cr 3+ and CO 2 due to their porous structure.

XRD analysis
From XRD pattern of HCP-AA (Fig. 8), we conclude that there is no sharp peak at 2θ and it has some noisy pattern, which suggests that it is amorphous in nature.

Pore size distribution
Pore size distribution of HCP-AA was examined.The results, extracted from Fig. 9, suggests that the pore width from ranges from 0.1 to 135 nm.It was measured by sorption analysis using nitrogen gas.

N 2 adsorption-desorption isotherms
The N 2 adsorption/ desorption isotherms of HCP-AA revealed the instantaneous uptake of N 2 gas at low pressure which suggests that abundant number of micropores are present while the gas uptake at moderate pressure is www.nature.com/scientificreports/attributed the presence of mesopores.Gas uptake at high pressure might be because of macropores as indicated in Fig. 10.

Thermogravimetric analysis
To further study the structural robustness and stability of HCP-AA, TGA was applied under inert nitrogen (N 2 ) gas atmosphere where temperature ranges of 20-800 °C as shown in Fig. 11.It signified that the HCP-AA was stable up to 400 °C due to highly cross-linked structure and abundance of nitrogen groups.The small mass loss under 100 °C could be the result of moisture and trapped solvent.www.nature.com/scientificreports/UV-Vis spectroscopy All hyper cross-linked polymers showed absorption in both ultraviolet and visible region.HCP-AA also absorbed light in UV and VIS region and it showed lambda max at 428 nm with absorbance 1.389, which is shown in Fig. 12.

Application of HCP-AA in removal of heavy metals
HCPs of different aromatic compounds are excellent adsorbents having remarkable properties for the uptake of an organic pollutant, dyes and the heavy metals due to their low cost, easy synthesis and greater surface area with large number of pores.A standard solution of 1000 mg L −1 of chromium metal was prepared by mixing 1.631 g of CrCl 3 to 1 L of distilled water.Standard solution of five different concentrations (10-50 mg L −1 ) was prepared with a gap of 10 mg L −1 , respectively.The solutions of lower concentrations are difficult to analyze         www.nature.com/scientificreports/using AAS and the results are not reliable 59 .The synthesized HCP-AA is grinded well and thoroughly washed with ethanol followed by water until pH 7.0 was obtained.This is done to make sure the removal of all possible impurities.After this 0.8 g of purified HCP-AA was added to each prepared standard solution.These conical flasks were placed on the rotary shaker and stir at 160 rpm at 298 K for 12 min.The solution was filtered and 230ATS atomic absorption spectrophotometer (AAS) was applied to determine the concentration of Cr 3+ from sample water.The AAS is a modern technique that is used for measurement of heavy metals in samples like the water.The percentage adsorption of HCP-AA for Cr 3+ was 93% at optimum conditions.To perform adsorption experiment on real water sample, sample was collected from Royal leather industries Limited Lahore, Pakistan.
The percentage adsorption of HCP-AA for Cr 3+ in real wastewater sample is 88%.Decrease in percentage removal is caused by interfering ions present in real water sample.The percentage removal was calculated by using Eq.(1).

FTIR analysis of HCP-AA before and after adsorption
Figure 13 shows the FTIR spectra of HCP-AA before and after an adsorption of Cr 3+ metal ions.The -OH group stretching peak appeared at 3055 cm −1 before adsorption, while after adsorption it is depressed and shifted to lower wavenumber (3045 cm −1 ), which may indicates the chemical adsorption of Cr 3+ metal ions by -OH group of HCP-AA.This peak is not completely disappear after adsorption that may indicates that the HCP-AA is reusable for adsorption.The other peaks such as C-Cl bending 715 cm −1 , C-H bending 839 cm −1 peaks merged, broaden and shifted to 687 cm −1 as chlorine may utilized its loan pair to adsorb Cr 3+ .While C-C stretching, C-N bending and C-O-H hydrogen bonding peaks were appeared in HCP-AA at 1122 cm −1 , 1213 cm −1 and 1314 cm −1 respectively, these functionalities peaks also broaden, merged together and centered at 1128 cm −1 to justify their role in adsorption.Similarly peak 1508 cm −1 belonged to C=C stretching and N-H bending at 1590 cm −1 were depressed and broaden at position 1560 cm −1 to elaborate the fact that N nitrogen loan pair and pi-electronic cloud of C=C played a vital role in adsorption phenomenon.Some small sharp peaks appeared between the 450-550 cm −1 were merged to form a single wide peak at point 542 cm −1 , which may indicates Cr and O interaction after the adsorption.

Mechanism of heavy metal adsorption
Nitrogen containing materials are attractive for the heavy metal ions uptake due to electron rich nature and there will be bond formation between electron deficient and electron rich centers (Lewis acid-base concept).
Pi-electronic cloud of benzene ring also form interactions with positively charged heavy metals due to cation-π interaction.Carboxyl group of HCP-AA after ionization form interactions with positively charged heavy metal ions.HCP-AA has porous surface so diffusion of heavy metals also takes place and it helps in adsorption of heavy metals.Reasonable surface area, narrow pore size distribution, hierarchical pore structure, accompanied by Lewis basic sites in HCP-AA are few of the desirable features for the study of heavy metal uptake where sorption isotherms measured at 273 K and 1 bar.The detailed mechanism of Cr 3+ adsorption on the surface of HCP-AA is given in Fig. 14.

Effect of particle size of HCP-AA on heavy metal adsorption
The HCP-AA was transformed into powdered form using a pestle mortar and subsequently separated through different mesh size screens to obtain distinct particle sizes: 0.5 mm and 1 mm.Initial experiments revealed that the HCP-AA particles with a size of 0.5 mm exhibited superior adsorption capabilities compared to those with (1) Figure 13.FTIR spectra of HCP-AA before and after an adsorption.
a 1 mm size.This can be attributed to the large surface area of the 0.5 mm particles.Given these findings, the 0.5 mm particle size was selected for subsequent experiments.

Point of zero charge (PZC)
The point at which surface of adsorbent has zero net charge, because the positive and negative charges are equal in numbers, this phenomenon is known as PZC.The aim of this PZC study was to investigate the pH at which the adsorbent surface exhibited an equal quantity of opposing ions and how adsorbent efficiency could be increased with the change of pH.A graph is produced between pHi and ∆pH to measure PZC 60 .Figure 15, displays the value of PZC, which was obtained from a line intersecting the x-coordinate.Trials for calculating PZC by using the salt addition technique revealed that at pH 4.0, the HCP-AA had a PZC.

Effect of wastewater pH
Results revealed that, pH of wastewater also affected the adsorption by HCP.In the experiment, the pH changed from acidic to basic (2-7) which showed that adsorption was not uniform throughout the pH changes.There will  www.nature.com/scientificreports/be a chance of precipitation of chromium ions at basic pH that is why the pH (2-7) was maintained for experiments to get reliable results 61 .The highest Cr 3+ adsorption took place at pH 7. Adsorption increased as the pH increased from point of zero charge, because at high pH, negative charge on adsorbent become dominant that caused strong electrostatic interaction between negatively charged surface and positive metal ion.Therefore, we conclude that optimum pH for adsorption by HCP takes place at pH 7 as revealed in Fig. 16.

Effect of adsorbent dose on adsorption
Various quantities of HCP-AA, from 0.1 to 1.0 g, were tried and put into different flasks, containing 50 ml of the sample solution.Initially, the percentage removal of Cr 3+ from water exhibited an upward trend as the quantity of adsorbent is raised (up to 0.8 g).The maximum Cr 3+ removal was 93% at 0.8 g after this it became constant.This phenomenon may be due to the increased accessibility of active sites and a greater surface area when high dose of adsorbent was employed.A minor increase in removal efficiency after 0.8 g was because of equilibrium between the adsorbent HCP-AA and heavy metals.Therefore, we conclude that optimum quantity of adsorbent HCP-AA is 0.8 g, which is shown in Fig. 17.

Effect of contact time
This study was done by differing the time of contact for adsorption from 1 to 12 min while keeping all other factors constant.At first, the removal percentage was increase at rapid rate in first 6 min due to the abundance of available empty spaces.After that, the rate of adsorption slowed down because lesser sites were available for adsorption of Cr 3+ .The maximum an adsorption took place at the time of 8 min, which was 93% and then became constant so, we conclude that optimum contact time for adsorption is 8 min as depicted in Fig. 18.

Langmuir isotherm
The Langmuir isotherm parameters are shown in Table 3.The parameter RL revealed an adsorption is favorable for these metals ions.These parameters were calculated by using the following relation; The RL value is between 0 and 1 which confirms the successful uptake of Cr 3+ .The Langmuir adsorption isotherm that is shown in Fig. 20 is excellent model to study monolayer adsorption and is mostly applied to find adsorption parameters for studies.
The linear form of the Langmuir equation is shown in Eq. ( 3).
(  www.nature.com/scientificreports/ The resulted data were well fitted by using Langmuir isotherm model 62 .

Freundlich isotherm
From the data obtained from Freundlich adsorption isotherm which is reported in Table 4 shows that 1/n = 0.731 while n = 1.37 shows an adsorption of Cr 3+ is favorable.Mathematical form of the Freundlich isotherm equation is given below; The Freundlich isotherm's linear form is given here; The Freundlich adsorption isotherm graph is shown in Fig. 21.
(3) P e q e = 1 q m•b = P e q m (4)

Temkin isotherm
Temkin isotherm experimental analysis is shown in the Table 5.The R 2 value of Temkin isotherm is 0.6654, which shows that this adsorption is favorable.

Mathematical form of Temkin isotherm model is following
Linear form of the Temkin model The Temkin adsorption isotherm graph is shown in Fig. 22.

Dubinin Radushkevich isotherm
Employing the Dubinin-Radushkevich isotherm, the mechanism of adsorption is expressed through Gaussian energy distribution onto a heterogeneous surface of the adsorbent.
This model aids in differentiating between the physical and chemical adsorption of heavy metal ions to calculate the mean free energy E per molecule of adsorbate.( 6) q e = q s exp (−K ad ε 2 ) (9) lnq e = ln q s − K ad ε 2  In the meantime, the value of ε can be determined as follows: where Ce, T, and R stand for the adsorbate equilibrium concentration (mg/L), absolute temperature (K), and gas constant (8.314J/mol K), respectively.The Dubinin-Radushkevich isotherm model is well known for the identification of its temperature-dependent feature, which is demonstrated when adsorption data at various temperatures are plotted as a function of the amount-adsorbed logarithm.lnq e versus ε 2 that is shown in Fig. 23, ε 2 is the square of potential energy, all suitable data will lie on the same curve, named as the characteristic curve.The constants such as q s , and K ad were calculated from the appropriate plot using equation No. 11 63 .From the linear plot of DRK model, q s was determined to 55.3 mg/g, the mean free energy, E = 0.7 kJ/mol indicates a physiosorption process with the R 2 = 0.5125.The comparison of isotherms parameters is illustrate in Table 6.The above data revealed that the adsorption can be best explained by Freundlich adsorption isotherm with R 2 = 0.9273.Study of isotherms revealed multilayer adsorption and adsorption capacity increases with increase in concentration of chromium ions.However close R 2 -value of Langmuir to Freundlich indicated monolayer adsorption with q max of 52.63 mg/g, which confirm heterogeneous nature of adsorbent surface.www.nature.com/scientificreports/Kinetics study Rate of adsorption was determined through kinetics study, which is discussed below, and it showed that an adsorption is well fitted with a pseudo second order model.

Pseudo first order reaction Linear equation of pseudo first order reaction is as follow
where Q e is equilibrium concentration, Q t is adsorption concentration at the time t and K is constant.Figure 24 describes the pseudo first kinetic model of adsorbent towards adsorption.

Pseudo second order reaction
Equation of the pseudo second order reaction is given below Figure 25 describes a pseudo second kinetic model behavior of adsorbent towards the adsorption.

Intraparticle diffusion model
Weber and Morris present an intraparticle diffusion model for finding out the adsorption process's diffusion mechanism and rate-controlling phase.This model's mathematical representation is as follows: where I is the layer's thickness, qt(mg/g) is the amount of adsorbate adsorbed at time "t" and kid is intra-particle diffusion constant.The plot produced by the data is linear, a value of regression coefficient, is 0.9582 64 .The value of Kdiff is 0.1052 and thickness of layer on surface of HCP-AA is 0.1186.The graph between t 1/2 and q t is shown in Fig. 26.
According to the comparative study it is observed that pseudo second order model is fit the best with R 2 = 0.979 as compares to pseudo 1st order kinetics model.The second order kinetic model predicts chemical adsorption of Cr 3+ on surface of polymer.The 1st order kinetic model's regression "R 2 " value "0.601" is lower than the ( 12)  www.nature.com/scientificreports/2nd order kinetic model's R 2 value 0.979.Additionally, q e calculated (0.601 mg g −1 ) value from Eq. 8 and 9 is closer to q e experimental value (0.589 mg g −1 ) as shown in Table 7.Therefore, second order kinetic model is the best to examine the kinetic constants of Cr 3+ adsorption on surface of HCP-AA.Comparative data analysis is shown in Table 7.

Thermodynamics study
The determination of heat changes in a system, or its state, can be described using various state functions, such as the Gibbs free energy (∆G), an entropy (∆S), and the enthalpy (∆H).These parameters offer insights into the mechanism of the adsorption, distinguishing between exothermic and endothermic reactions.The thermodynamic relationships can be expressed through the equation:   • ΔH denotes enthalpy • T is a temperature in Kelvin • K c is an equilibrium constant An equilibrium constant (K c ) is defined as the ratio of a amount of heavy metal adsorbed on surface of HCP-AA at the equilibrium (C a ) to equilibrium concentration of a heavy metal in the solution (C e ), as given by: In exothermic processes, the Gibbs free energy (ΔG 0 ), a entropy (ΔS 0 ), and the enthalpy (ΔH 0 ) exhibit negative values, governed by the relationship: Negative enthalpy and entropy value signify the exothermic nature of the reaction, suggesting an inverse relationship with temperature.As temperature increases, adsorption tends to decrease, and vice versa.The graph between ln Kc versus 1/T for the adsorption of chromium over HCP-AA is illustrated in Fig. 27.The value for adsorption enthalpy (ΔH 0 ) was − 60.91 kJ/mol and adsorption entropy (ΔS 0 ) was − 45.79 kJ/mol K which were calculated from plot.The resulting values for Gibbs free energy (ΔG 0 ) are presented in Table 8.The Positive ΔG 0 suggests the non-spontaneous nature of a process.A rise in the ΔG 0 values with increase in temperature shows adsorption is unfavorable at the higher temperatures.A negative ΔH 0 value implies that the adsorption of Cr 3+ is exothermic, while the negative ΔS 0 value shows a reduction in unpredictability at the contact between the adsorbent-adsorbate during the adsorption.
The adsorption capacity of HCP-AA in relation to other comparable adsorbents is displayed in Table 9.It shows 52.63 mg/g adsorption capacity of HCP-AA that is due to is high surface area, porous surface and active functional groups.
Application of HCP-AA for CO 2 uptake Pure CO 2 gas was provided through a cylinder, connected with a heater.The pressure and temperature of gas was controlled till it was transferred to reactor containing adsorbent (HCP-AA) for the adsorption experiment.( 16)  www.nature.com/scientificreports/ The electrical heater was used to heat the reactor and thermocouple was used to regulate the temperature.Temperature and pressure of CO 2 were determined at 273 and 298 K by using thermocouple and pressure gauge connected to the computer respectively.Nitrogen gas was used for as an inert medium for discharging and cleaning of mixing tank represented in Fig. 28 71 .

Mechanism of adsorption of CO 2
The HCP-AA showing high BET surface area, abundant pores, carboxyl (-COOH) and amino (-NH 2 ) group surged us to investigate their CO 2 adsorption properties.Nitrogen rich HCPs were used extensively for the CO 2 adsorption because of polymers host-guest chemistry.
There are two ways that HCP-AA can adsorb CO 2 : first, the amine group adsorbed the CO 2 molecules by the creation of a zwitterion intermediate (R-NH +2-COO − ).A zwitterion intermediate then donates its H + to the nearby amine group to generate ammonium-carbamate ion pairs ((R-NH 3+ -COO-NH-R)), and intermolecular H + transfer can also form carbamic acid (R-NH-COOH) species 72 .General mechanism for the CO 2 adsorption by HCP-AA through chemisorption is shown in Fig. 29.

CO 2 adsorption
A CO 2 adsorption study of HCP-AA were conducted at 273 and 298 K is shown in Fig. 30.The adsorption isotherms demonstrating the HCP-AA ability to the CO 2 uptake at various temperatures.
Quantity of CO 2 adsorbed on surface of HCP-AA is directly related to pressure applied.As, we increased pressure, the quantity of CO 2 adsorbed increased as shown in Fig. 30.HCP-AA adsorbed 31.1 cm 3 /g CO 2 at pressure 850 mmHg at 273 K but at temperature 298 K, it adsorbed 25.2 cm 3 /g at 850 mmHg.The adsorption capacity of the HCP-AA for CO 2 was a 1.39 mmol/g at 273 K and 1.12 mmol/g at 298 K, which is calculated by using a following relation.
It showed a maximum CO 2 uptake ability of 6.1 wt% at 273 K and 5 wt% at 298 K was because of its high surface area, abundant amino and the carboxyl groups as shown in Fig. 31.The CO 2 uptake ability reduced at higher temperatures.
To clarify the interaction of HCP-AA with CO 2 molecule, the isosteric heat was estimated by using Clausius-Clapeyron equation.The Q st of the HCP-AA (29.2-25.4kJ mol −1 ) which is shown in Fig. 32 is because of tightly packed porous structure and nitrogen content.(18)  Adsorption capacity = moles of CO 2 adsorbed/mass of adsorbent in grams × 1000 www.nature.com/scientificreports/A comparison of CO 2 uptake capacity of HCP-AA with other similar adsorbents at 1 bar pressure is shown in Table 10.It shows the adsorption capacity of 1.39 and 1.12 mmol/g at 273 K and 298 K, respectively, which is due to the high surface area, porous surface and the nitrogen content.3+ and CO 2 uptake For industrial applications, reusability is the important attribute due to economic value.Conducting repeated adsorption on the same HCP-AA up to 10 cycles reveal that there is a minimal decrease in its efficiency till 3     cycles after that HCP-AA start occupying with heavy metal ions so there is gradually decrease in its efficiency to adsorb further metal ions.For reuse of HCP-AA for several times acidic medium is provided to the HCP-AA by treated it with 0.1 molar HCl solution so desorption takes place but this can regenerate the capability of HCP-AA for some extent.The graph of effect of repeated adsorption experiment on adsorption is shown in Fig. 33.

Reusability of HCP-AA for Cr
To study the adsorbents recyclability for CO 2 uptake, the ten adsorption cycles were performed at the 298 K and HCP-AA were recycled for 8 h at 410 K in a vacuum oven.The HCP-AA adsorption potential reduced by 1.5% after 10 cycles.

Conclusions and future perspectives
There is no life without plenty of fresh water and air, but due to industrialization and population bloom, clean sources of water are declining day-by-day and air is polluted with greenhouse gases like CO 2 , which can result in global warming.The situation is getting worse in developing countries with few reservoirs of fresh water.One of the main cause of water pollution are heavy metals, which may cause fatal diseases in humans.Therefore, it is an ultimate need to purify water resources.HCPs are excellent candidates to clean water and air through their adsorption capacities as per their structural features like porosity and high surface area.In this article, a simple approach for the synthesis of hyper cross-linked polymer (HCP-AA) through Friedel-Crafts reaction and its use for sequestration of CO 2 and Cr 3+ metal ions with potent results are reported.The produced HCP-AA contains oxygen and nitrogen, which gives them a great selectivity and high adsorption capacity for the pollutants along with high stability and reusability.The designed HCP-AA can be a good candidate to solve today's world problems like global warming and water scarcity.In near future, HCPs have the potential to use in industries and powerhouses, where significant amount of heavy metals, dyes and CO 2 are emitted.In order to keep environment clean and safe it is better to use such types of HCPs, before the release above-mentioned environmental pollutants to the environment.In order to make it more effective, further work is required to make such HCPs synthesis more feasible, optimized, efficient, and cost effective.

Figure 3 .
Figure 3. Possible reaction mechanism for synthesis of HCP-AA.

Figure 14 .
Figure 14.Mechanism for an adsorption of Cr 3+ on surface of HCP-AA.

Figure 16 .
Figure 16.Effect of pH on adsorption.

Figure 17 .
Figure 17.Effect of quantity of HCP-AA on adsorption.

Figure 18 .
Figure 18.Effect of contact time on adsorption.

Figure 19 .
Figure 19.Effect of temperature on adsorption.

Figure 29 .
Figure 29.Mechanism of adsorption of CO 2 on HCP-AA.

Figure 32 .
Figure 32.Heat of adsorption of HCP-AA for different quantities of CO 2 .

Table 1 .
Physical properties of HCP-AA.

Table 2 .
EDX results showing the percentage composition of HCP-AA.

Table 3 .
Langmuir Isotherm data for chromium metal.

Table 4 .
Experimental data of Freundlich isotherm.

Table 5 .
Experimental data of Temkin isotherm.

Table 6 .
Comparison of adsorption isotherm parameters.

Table 7 .
Comparison of kinetics graph.

Table 8 .
Thermodynamics study of adsorption.

Table 9 .
Comparison of HCP-AA with other adsorbents for chromium removal.

Table 10 .
Comparison of HCP-AA performance with other adsorbents.